1. Field of the Invention
The present invention relates to a battery fuel gauge circuit.
2. Description of the Related Art
Various kinds of battery-driven electronic devices such as cellular phone terminals, digital still cameras, PDAs (Personal Digital Assistants), laptop personal computers, etc., each include electronic circuits such as a CPU (Central Processing Unit) and a DSP (Digital Signal Processor), which are each configured to perform digital signal processing, a liquid crystal panel, other kinds of analog circuits and digital circuits, and so forth. Such electronic circuits are configured to operate receiving an electric power supply from a battery.
Such an electronic device has a function of detecting the remaining battery charge by measuring a current (which will be referred to as the “battery current” hereafter) supplied from the battery to a load, and by integrating the current values thus measured. Such a function is provided by a battery fuel gauge circuit, which is also referred as the “coulomb counter”.
Description will be made below regarding the detection of the remaining battery charge for an electronic device such as a cellular phone terminal and so forth having a communication function. FIG. 1 is a diagram which shows an example of the waveform of the battery current IBAT in a standby state (sleep state) of a cellular phone terminal. It should be noted that the vertical axis and the horizontal axis shown in the waveform diagrams and the time charts in the present specification are expanded or reduced as appropriate for ease of understanding. Also, each waveform shown in the drawings is simplified for ease of understanding.
Such a cellular phone terminal is configured to periodically perform packet transmission (TX) and packet reception (RX) between itself and a base station for the purpose of location registration and so forth. In the transmission period TTX, a large current flows through a transmission circuit including a power amplifier, a modulator, and a baseband IC (Integrated Circuit), leading to an increase in the battery current IBAT. In this period, such a battery current IBAT changes dynamically according to the distance between the cellular phone terminal and the base station. Also, in the reception period TRX, a receiving circuit, which includes a low-noise amplifier, a demodulator, a baseband IC, and so forth, is operated, leading to an increase in the battery current IBAT. In contrast, in a period (non-communicating period) in which communication is not made in the standby state, almost all the internal circuit blocks of the cellular phone terminal are set to the standby state. In this state, the battery current IBAT becomes very small, and there is almost no change in the battery current IBAT.
In order to integrate, with high precision, a burst current that flows intermittently during the transmission period TTX and the reception period TRX, there is a need to increase the sampling frequency at which the battery current IBAT is to be sampled, so as to improve the resolution in the time-axis direction. However, such an increased sampling frequency leads to an increase in the power consumption of the battery fuel gauge circuit, which is undesirable.